Industrial process control and monitoring systems are used to monitor and/or control industrial processes. For example, a process variable such as pressure, temperature, flow, etc., of a process fluid can be measured by a process variable transmitter. This information allows an operator to monitor operation of the process. Further, the measured process variable can be used as an input to a control algorithm and used to control operation of the process.
In many instances, the process variable transmitter is located at a remote location and transmits information back to a central location over a process control loop. The process control loop can include, for example, a two wire process control loop in which the process variable is transmitted in an analog manner, for example, based upon a 4-20 mA current level flowing through the loop or a digital manner to the central location. The same two wires can be used to provide power to the process variable transmitter. Another example process control loop is a wireless control loop in which data is transmitted wirelessly.
One type of process variable which is measured is temperature. Various types of temperature sensors are used to measure temperature. One type of temperature sensor is a resistance based temperature sensor known as an RTD. The resistance of the RTD varies as a function of temperature. Typically, the resistance is accurately measured using a Kelvin connection to the RTD in which a first pair of wires carry a current and a second pair of wires are used to measure a voltage drop across the RTD. If one of the connections degrades, inaccurate temperature measurements may be obtained and maintenance must be performed.
RTD sensors are thus temperature-sensing devices in which the resistance of an RTD sensor increases with temperature. Regardless of the type of wiring configuration, at times, there may be poor connections on any of the terminals wires, which can lead to measurement errors. In the case of a 3 or 4 wire RTD, when one or more of the 3 or 4 wires of the RTD breaks, the temperature measuring field transmitter enters a failsafe condition and no temperature measurement is provided until the break is rectified. Such as a sensor wire break can negatively impact plant operations, for example, until the break is repaired or dealt with in some manner.